The present invention relates to a circuit for detecting distortion in an amplifier, and in one embodiment, an audio amplifier.
As is known, during normal operation of an audio amplifier, excessively strong signals applied at the input can cause auditive noise, or can damage the loudspeaker, when the amplifier reaches the maximum dynamic level and is saturated.
In order to solve this problem, a digital or analogue feedback circuit is normally provided and reduces the input signal when the output signal reaches a certain percentage of distortion. For this purpose, it is necessary to provide a distortion detector, which detects when the audio amplifier is in a distortion condition and generates a corresponding control signal for the feedback circuit. However, it has been found that the feedback circuit must not intervene immediately when the distortion occurs, since this reduces the output power perception; acoustic tests have shown that a detector with an intervention threshold for distortion of 5-10% is a good compromise between the output power perception, and the start of noise caused by saturation.
A known distortion detector, for example described in U.S. Pat. No. 5,068,620, is based on comparing the signals applied to the positive and negative inputs of the amplifier. When there is no distortion, the signals at the inputs of the amplifier are equal, and the distortion detector does not intervene; when the amplifier is causing distortion, the amplitude of the negative input is stabilized, whereas that of the positive input continues to increase; as soon as the difference between the signals reaches a certain value, a distortion signal is generated and is used to reduce the input signal.
In this known distortion detector, it is disadvantageous that the distortion detection threshold is dependent on the supply voltage, such that the accuracy of the detector depends on the supply voltage, as well as on the saturation of the amplifier power elements, on the temperature, and on the load. In addition, two comparators are necessary, one for each phase of the input signal.
According to the embodiments of the present invention, an amplifier and a method are provided for detecting distortion in an amplifier are provided. In accordance with one embodiment, an amplifier is provided that includes a first input, a first output supplying a first output signal; a first feedback network connected between the first input and the first output; a distortion detection circuit; the first feedback network having a first and a second feedback element arranged in series to form a first intermediate node supplying an intermediate signal that is in phase with the output signal when in the absence of distortion and that is in phase opposition when in the presence of distortion; and the distortion circuit having a phase comparator for detecting the phase of the output signal and the intermediate signal and generating a distortion-indicative signal when the intermediate signal is in phase opposition with respect to the output signal.
In accordance with a method of the disclosed embodiments of the present invention, detecting distortion in an amplifier having an input and an output supplying an output signal includes connecting a first and a second feedback element between the input and the output in series with each other, detecting an intermediate signal that is present between the first and second feedback elements, and generating a distortion-indicative signal when the intermediate is in phase opposition with respect to the output signal.